Alright...yonin is four people right? But how do you say there is four of something else? I've taken Mandarin in school before and we have to use all kinds of 'measure words'. Is there anything like that in Japanese?

Four is just an example. I would appreciate a more general answer if ya have one!

To say stuff like "four months" you would add a small katakana ka between the number and object. IE yon ka getsu would be four months. I think this applies to most things when you count, but it may only apply when dealing with time or length.